Method and system for determining a thickness of a layer

ABSTRACT

The present invention relates to a method of determining a thickness of at least one layer on at least one semiconductor wafer ( 12 ), comprising the steps of: projecting a first laser pulse ( 14 ) on a surface ( 16 ) of the at least one layer ( 10 ), thereby generating an acoustical wave due to heating of the surface of the at least one layer ( 10 ); after a propagation time of the acoustical wave, projecting a series of second laser pulses ( 18 ) on the surface ( 16 ) of the at least one layer ( 10 ); measuring reflected laser pulses ( 20 ) of the second laser pulses ( 18 ), thereby sensing the times of reflection property changes of the surface ( 16 ) of the at least one layer ( 10 ); and determining the thickness of the at least one layer ( 10 ) by analyzing the times of reflection property changes. The present invention further relates to a system for determining a thickness of a layer ( 10 ) on a semiconductor wafer ( 12 ).

FIELD OF THE INVENTION

[0001] The present invention generally relates to a method ofdetermining a thickness of a layer on a semiconductor wafer, and moreparticularly to a method of determining the thickness of opaque layers.The present invention further relates to a system for determining athickness of a layer on a semiconductor wafer.

BACKGROUND OF THE INVENTION

[0002] There are several semiconductor wafer processing techniques inwhich thin layers or films are deposited and/or removed. Thesetechniques could be improved by the knowledge of the layer thickness.Frequently, optical techniques are used to determine the layerthickness. However, these techniques only work for measuring transparentlayers.

[0003] For example, electroplating is an emerging process technologybeing used to facilitate the deposition of copper films to integratedcircuit structures. The technology of electroplating is currentlylimited by the lack of process control mechanisms including depositiontermination. The process is typically performed with timed process stepswhich does not guarantee a deposition with accurate thickness.

[0004] As another example, plasma etch tool technology is currentlylimited by endpointing schemes that are not directly based on filmthickness targets but on indirect methods. Such indirect methods are,for example, optical emission spectroscopy, or timing methods that arebased on previously run test wafers. Further, as the film thickness isdecreasing, interferometry may be used as an indirect endpointdetermination.

[0005] Also wet tool technology is currently limited by endpointingschemes that are not directly based on film thickness targets. Also inthis case, timed methods based on previously run test wafers are used.

[0006] Similarly, in diffusion process technology, indirect methods areused. Also in this case, interferometry, or timed methods based onpreviously run test wafers are employed.

[0007] Further technologies in which the thickness of layers on a waferis of interest are chemical vapor deposition (CVD) and physical vapordeposition (PVD). Also in these processes, the current measurementtechnology is limited by a lack of control mechanisms.

[0008] The present invention seeks to solve the above mentioned problemsby providing a new method and a new system for reliably and accuratelydetermining a thickness of a layer on a semiconductor wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 shows two schematic cross sectional views of semiconductorwafers with different layers, illustrating the present invention;

[0010]FIG. 2 is a cross sectional view of a plasma etch chamber,illustrating a system according to the present invention;

[0011]FIG. 3 is a top view of a wet processing tank, illustrating asystem according to the present invention;

[0012]FIG. 4 is a top view of a wet processing tank, illustrating afurther system according to the present invention;

[0013]FIG. 5 is a side view of a wet processing arrangement illustratinga further system according to the present invention;

[0014]FIG. 6 is a side view of a diffusion tube, illustrating a furthersystem according to the present invention;

[0015]FIG. 7 is a side view of a chamber representing a chemical vapordeposition chamber or a plasma etch chamber, illustrating furthersystems according to the present invention;

[0016]FIG. 8 is a side view of a chamber according to FIG. 7,illustrating a process step of a method according to the presentinvention;

[0017]FIG. 9 is a side view of a chamber according to FIGS. 7 and 8,illustrating a further process step of a method according to the presentinvention;

[0018]FIG. 10 is a side view of a physical vapor deposition sputterchamber, illustrating a further system according to the presentinvention;

[0019]FIG. 11 is a side view of a physical vapor deposition sputterchamber according to FIG. 10, illustrating a process step of a methodaccording to the present invention;

[0020]FIG. 12 is a side view of a physical vapor deposition sputterchamber according to FIGS. 10 and 11, illustrating a further processstep of a method according to the present invention;

[0021]FIG. 13 is a side view of a process chamber and a transferchamber, illustrating a further system according to the presentinvention; and

[0022]FIG. 14 is a flow diagram illustrating a method according to thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0023] According to the present invention, a method of determining athickness of at least one layer 10 on at least one semiconductor wafer12 is provided, comprising the steps of:

[0024] projecting a first laser pulse 14 on a surface 16 of the at leastone layer 10, thereby generating an acoustical wave due to heating ofthe surface 16 of the at least one layer 10,

[0025] after a propagation time of the acoustical wave, projecting aseries of second laser pulses 18 on the surface 16 of the at least onelayer 10,

[0026] measuring reflected laser pulses 20 of the second laser pulses18, thereby sensing the times of reflection property changes of thesurface 16 of the at least one layer 10, and

[0027] determining the thickness of the at least one layer 10 byanalyzing the times of reflection property changes.

[0028] According to the present invention, there is further provided asystem for determining a thickness of at least one layer 10 on at leastone semiconductor wafer 12, comprising

[0029] means 22, 28, 30 for projecting a first laser pulse 14 on asurface 16 of the at least one layer 10, thereby generating anacoustical wave due to heating of the surface 16 of the at least onelayer 10,

[0030] means 24, 28 for projecting a series of second laser pulses 18 onthe surface 16 of the at least one layer 10,

[0031] means 30 for measuring reflected laser pulses 20 of the secondlaser pulses 18, thereby sensing the times of reflection propertychanges of the surface 16 of the at least one layer 10, and

[0032] means for determining the thickness of the at least one layer 10by analyzing the times of reflection property changes.

[0033] Acoustical wave techniques use a first laser pulse to produce afine spot of instantaneous heating on the wafer surface. This produces alocalized sound wave that propagates through the film layer. When aninterface is reached, a partial echo signal is returned. A portion ofthe sonic wave crosses the interface and it propagates through theunderlying film layer. This process continues through all layers. Eachecho wave returning to the surface changes the surface reflectionproperty. This reflection property change is measured by the secondlaser pulses. These probe pulses may be diverted from the first laserpulse by a beam splitter. After detection of the second laser pulses, asoftware analysis of the signal converts the time between soundgeneration and echo wave detection into an accurate film thicknessvalue. Multi-layer film stacks can be measured individually orsimultaneously. An important advantage of this technique is related tothe possibility to measure opaque film layers deposited on a wafer thatcannot be measured optically. Additionally, the laser beam can be movedto multiple locations across the wafer to measure within-waferuniformity.

[0034] The described technique may be used to measure the thickness offilms deposited by electroplating. It is possible to measure the filmthickness in solutions comprising plating materials, such as copper.Such electroplating is performed, for example, by depositing each waferindividually by immersing in a solution and applying electric current.The measurement technique can be implemented when the wafer is removedfrom the solution. Based on the measurement results, additionaldepositions can be performed. The system according to the presentinvention can be used as an endpointing system, if it is employeddirectly in the deposition solution. In this case, the measurementtechnique could be referenced using a separate tank with similarsolutions without wafers to subtract out the contribution of the liquidto the measurement result. Such a reference tank may also be used fordetermining the properties of the solution. This could be achieved byemploying a reference tank which is not used for processing.

[0035]FIG. 1 shows two schematic cross sectional views of semiconductorwafers with different layers, illustrating the present invention. FIG. 2is a cross sectional view of a plasma etch chamber, illustrating asystem according to the present invention.

[0036] In the upper part of FIG. 1 a semiconductor wafer 12 is shown ata first time during plasma etch. In the lower part of FIG. 1 thesemiconductor 12 is shown at a second time; after the first time. Thesemiconductor 12 has a layer 10 that is to be removed by plasma etching.A further structure 56 is provided on the layer 10. Below this structure56 the layer 10 is maintained. On the surface 16 of the layer 10 a firstlaser pulse 14 is projected. Thereby, an acoustical wafer is generateddue to heating of the surface 16. A second laser pulse 18 is projectedon the surface 16, and it is reflected. The reflected laser pulse 20 maybe detected. Since the reflection property changes of the surface 16depend on the thickness of the layer 10, the thickness of the layer 10may be determined by measuring the reflection property changes.

[0037] In FIG. 2 an plasma etch process chamber 32 with a window 36 forthe first laser pulse, a window 38 for the incoming second laser pulseand a window 40 for the reflected second laser pulse 20 is provided.Within the plasma etch process chamber the plasma 34 is established. Dueto the set-up, an in-situ measurement of the thickness of layer 10 ispossible.

[0038] The described measurement method for plasma etch chambers isdesigned to work on areas with and without topography. The methodeliminates the need for test wafers and errors associated with targetingdeposition times on unpatterned wafers versus wafers with topography andfeatures. Another achievable measurement feature is the profiling ofside wall deposition in the chamber. Also, processes that remove acertain portion of a film layer while stopping in the same layer, suchas recess etches, could benefit from the method and system according tothe present invention.

[0039] The acoustical wave metrology allows measurement of differentopaque films, such as metals, individual oxides or dielectric films, andon film-stacks consisting of oxides and metals. The sample area could bechosen precisely using a pattern recognition system or scan area.Photoresist selectivity could also be monitored in real time during theplasma etch processing. Processes could be modified to respond to theresults of measurements, such as triggering overetch steps forendpointing, or reduction in parameters to slow etching to achieveprecise removal targets.

[0040]FIG. 3 is a top view of a wet processing tank 42, illustrating asystem according to the present invention. In a wet processing tank 42 aplurality of wafers 12 is arranged. Only one of these wafers 12 ismonitored by the system and method according to the present invention.Through a window 36 a first laser pulse 14 is projected on the firstwafer 12. Through another window 38 a series of second laser pulses 18is projected. Both laser pulses may be projected from different lasers22, 24. Alternatively, the laser pulses 14, 18 may be generated from thesame laser and diverted by a beam splitter. The reflected second laserpulse 20 propagates through a third window 40 or view port, and it isdetected by a detector 26.

[0041]FIG. 4 is a top view on a wet processing tank 42, illustrating afurther system according to the present invention. In this set-up also aplurality of wafers 12 is arranged in a wet processing tank 42. However,more than one wafer 12 can be monitored. To achieve this, a quartz boat44 and mirrors 46 are used to transmit the laser beams 14, 18, 20. Thefunctional block 30 may operate as a laser, so that the first laserpulse may be projected from both sides on the wafers 12. Duringtransmission of the second laser pulses 18, the functional block 30operates as a detector for measuring the reflected second laser pulses20.

[0042]FIG. 5 is a side view of a wet processing arrangement,illustrating a system according to the present invention. A boat 50containing the wafers 12 is removed from the wet processing tank 42 inorder to perform a method according to the present invention. Ameasurement system 48 is provided that can be moved over the wafers 12.All wafer locations can be measured. It is again possible to project thefirst laser beam 14 from both sides on the wafers. The second laserpulses 18 are projected from a laser source 28, and the reflected pulses20 are detected by a detector 30. Additionally, a N₂ supply 52 isprovided for drying the measurement area.

[0043] The wet tool arrangements that are described with reference toFIGS. 3 to 5 preferably use wet solutions consisting primarily of acids.The system and method according to the present invention work insolutions that remove significant layers of material from the wafersurface. It is possible to provide a reference by a separate tank usingdeionised (DI) water to subtract out the contribution of the liquid tothe measurement. Additionally, the liquid properties, such as density,can be measured with reference to the DI water tank with no wafersinside. This could be done with a reference tank of the acid or etchantsolution which does not contain wafers and which is not used forprocessing, the signal information being stored in a processor as areference. The measurement of the liquid properties may be used todetermine the composition, for example the concentration of the etchantor acid.

[0044]FIG. 6 is a side view of a diffusion tube 54 illustrating afurther system according to the present invention. In a quartz diffusiontube 54 a plurality of wafers is arranged. The laser beams are directedthrough a quartz boat 44 and by mirrors 46, similarly to the embodimentaccording to FIG. 4. Also the arrangement of the laser sources 28, 30and the detector 30 is similar to the arrangement of the embodimentaccording to FIG. 4.

[0045] Also in the case of diffusion process technology, the system andthe method according to the present invention work on areas with andwithout topography. The invention can be applied on all film depositiondiffusion processes. Additional information besides the thickness wouldbe the composition and density of the film or the individual layers of afilm stack. The method according to the present invention eliminates theneed for test wafers and errors associated with targeting depositiontimes from unpatterned wafers versus products, i.e. wafers withtopography and features. The method according to the present inventioncan be performed from inside or outside the diffusion tube, or withinthe tube in a shielded apparatus. The edges of stacked wafers can bemeasured. The system can be angled or scanned by system movement toperform measurements by zone, or ultimately all wafers if desired. It isalso possible to perform a correlation from outside the tube to subtractout the effect of the diffusion tube by using a reference signalapproach. For rapid thermal processes (RTP) the lamps can be switched offor the measurement, before the process is determined. An additionaldeposition can be done before the processing is completed.

[0046]FIG. 7 is a side view of a chamber 58 representing a chemicalvapor deposition chamber or a plasma etch chamber, illustrating furthersystems according to the present invention. Inside the chamber 58 areactive gas 60 is provided from a shower head 64. A chemical reactionproduct that is generated in a gas mixing area 66 is deposited on awafer 12 located on a pedestal 62. The gas mixing may be plasmaassisted. In addition to conventional chambers, windows 38, 40 areprovided, the function of which is described further below.

[0047]FIG. 8 is a side view of a chamber 58 according to FIG. 7,illustrating a process step of a method according to the presentinvention. A first laser pulse 14 from a laser source 22 reaches thewafer 12 through a port (not shown) in the shower head 64, therebycreating an acoustical wave according to the present invention.

[0048]FIG. 9 is a side view of a chamber 58 according to FIGS. 7 and 8,illustrating a further process step of a method according to the presentinvention. A series of second laser pulses 18 from a laser source 24 isprojected on the wafer 12 through a window 38 in the chamber 58. Bymeasuring the reflected laser pulses 20 that reach a detector 26 througha window 40 in the chamber 58 the times of reflection property changesof the wafer surface are sensed. Thus, the thickness and/or physical andchemical properties of at least one layer on the wafer surface can bedetermined.

[0049]FIG. 10 is a side view of a physical vapor deposition (PVD)sputter chamber 68, illustrating a further system according to thepresent invention. Inside the PVD sputter chamber 68 a plasma 34 isprovided. Ions 70 from the plasma 34 strike the metal to be depositedand the sputtered metal 72 is deposited on the wafer 72. In addition toconventional PVD sputter chambers, windows 38, 40 are provided, thefunction of which is described further below.

[0050]FIG. 11 is a side view of a physical vapor deposition sputterchamber 68 according to FIG. 10, illustrating a process step of a methodaccording to the present invention. A first laser pulse 14 from a lasersource 22 is split by a beam splitter 74, reflected by a mirror 76 and amirror 78, respectively, and it reaches the wafer 12 through a window 38and a window 40, respectively, thereby creating an acoustical waveaccording to the present invention.

[0051]FIG. 12 is a side view of a physical vapor deposition sputterchamber 68 according to FIGS. 10 and 11, illustrating a further processstep of a method according to the present invention. The process step issimilar to the process step described with reference to FIG. 9.

[0052]FIG. 13 is a side view of a process chamber 58, 68 and a transferchamber 80, illustrating a further system according to the presentinvention. The process chamber 58, 68 can be, for example, a CVD, plasmaetch, PVD, or RTP chamber. The wafer 12 has been extracted from theprocess chamber 58, 68 by a robotic handling system 84. Thus, themeasuring method according to the present invention can be performedoutside the process chamber 58, 68. The first laser pulse 14 from thelaser source 22 and the series of second laser pulses 24, 26 from thelaser source 24 are projected through the same window 82 or view port.Also the reflected laser pulses 20 reach the detector 26 through thesame window 82. There are further embodiments within the scope of thepresent invention in which the transfer chamber comprises more than onewindow for the distinct laser pulses.

[0053] There are several advantages related to the external measuringsystem and method according to FIG. 13. Many systems already have viewports on the transfer chamber 80. For example, it is possible to use oneof these view port locations for multiple process chambers 58, 68.Further, no complications occur due to an interference of the measuringdevice and the process. Moreover, the wafer 12 can be kept in vacuum sothat re-processing is possible. Additionally, by using the movements ofrobot stepper motors for the robotic handling system 84, across-waferuniformity can be measured.

[0054]FIG. 14 is a flow diagram for illustrating a method according tothe present invention. In a first step S01 a first laser pulse isprojected on the surface of a layer on a semiconductor wafer. Thereby,the surface is locally heated and an acoustical wave is generated. Theacoustical wave propagates through the layer and is partially reflectedat interfaces to neighboring layers. The echo that returns at thesurface of the upper layer changes the reflection property of the wafer.In order to sense the reflection property, in step S02 second laserpulses are projected on the surface of the semiconductor wafer. In stepS03 the reflection property changes are sensed by measuring thereflected second laser pulses. In step S04 the thickness of the layer isdetermined by analyzing the times of reflection property changes.

[0055] While the invention has been described in terms of particularstructures, devices and methods, those of skill in the art willunderstand based on the description herein that it is not limited merelyto such examples and that the full scope of the invention is properlydetermined by the claims that follow.

1. A method of determining a thickness of at least one layer on at leastone semiconductor wafer, comprising the steps of: projecting a firstlaser pulse on a surface of the at least one layer, thereby generatingan acoustical wave due to heating of the surface of the at least onelayer, after a propagation time of the acoustical wave, projecting aseries of second laser pulses on the surface of the at least one layer,measuring reflected laser pulses of the second laser pulses, therebysensing the times of reflection property changes of the surface of theat least one layer, and determining the thickness of the at least onelayer by analyzing the times of reflection property changes.
 2. Themethod according to claim 1, wherein the first laser pulse is generatedby a first laser source, and the series of second laser pulses isgenerated by a second laser source.
 3. The method according to claim 1,wherein the first laser pulse and the series of second laser pulses aregenerated by the same laser source.
 4. The method according to claim 1,wherein the first laser pulse and the series of second laser pulsespropagate on different light paths between a source and the at least onesemiconductor wafer.
 5. The method according to claim 1, wherein thefirst laser pulse and the series of second laser pulses propagate onessentially the same light path between a source and the at least onesemiconductor wafer.
 6. The method according to claim 1, wherein thethickness of layers in multi-layer film stacks is measured.
 7. Themethod according to claim 1, wherein multiple locations across thesemiconductor wafer are measured to determine wafer uniformity.
 8. Themethod according to claim 1, wherein the at least one layer is depositedby electroplating.
 9. The method according to claim 1, wherein thethickness of the at least one layer is determined to monitor a plasmaetch process.
 10. The method according to claim 1, wherein the thicknessof the at least one layer is determined to monitor a diffusion process.11. The method according to claim 1, wherein the thickness of the atleast one layer is determined to monitor a chemical vapor depositionprocess.
 12. The method according to claim 1, wherein the thickness ofthe at least one layer is determined to monitor a physical vapordeposition process.
 13. The method according to claim 1, wherein thethickness of the at least one layer is determined to monitor a wetprocess.
 14. A system for determining a thickness of at least one layeron at least one semiconductor wafer, comprising means for projecting afirst laser pulse on a surface of the at least one layer, therebygenerating an acoustical wave due to heating of the surface of the atleast one layer means for projecting a series of second laser pulses onthe surface of the at least one layer, means for measuring reflectedlaser pulses of the second laser pulses, thereby sensing the times ofreflection property changes of the surface of the at least one layer,and means for determining the thickness of the at least one layer byanalyzing the times of reflection property changes.
 15. The systemaccording to claim 14, further comprising a first laser source forgenerating the first laser pulse, and a second laser source forgenerating the series of second laser pulses.
 16. The system accordingto claim 14, further comprising a laser source for generating the firstlaser pulse and the series of second laser pulses.
 17. The systemaccording to claim 14, wherein the first laser pulse and the series ofsecond laser pulses propagate on different light paths between a sourceand the at least one semiconductor wafer.
 18. The system according toclaim 14, wherein the first laser pulse and the series of second laserpulses propagate on essentially the same light path between a source andthe at least one semiconductor wafer.
 19. The system according to claim14, wherein the thickness of layers in multi-layer film stacks ismeasured.
 20. The system according to claim 14, wherein multiplelocations across the semiconductor wafer are measured to determine waferuniformity.
 21. The system according to claim 14, wherein the at leastone layer is deposited by electroplating.
 22. The system according toclaim 14, wherein the thickness of the at least one layer is determinedto monitor a plasma etch process.
 23. The system according to claim 14,wherein the thickness of the at least one layer is determined to monitora diffusion process.
 24. The system according to claim 14, wherein thethickness of the at least one layer is determined to monitor a chemicalvapor deposition process.
 25. The system according to claim 14, whereinthe thickness of the at least one layer is determined to monitor aphysical vapor deposition process.
 26. The system according to claim 14,wherein the thickness of the at least one layer is determined to monitora wet process.